Agglutination inhibition assays provide a sensitive way to detect small amounts of antigens. Originally, home pregnancy tests used latex particles coated with hCG and anti-hCG antibodies - a pregnant woman's urine would contain hCG which inhibited agglutination, indicating pregnancy. Agglutination inhibition can also detect illegal drugs. It is widely used in clinical labs to determine exposure to viruses by checking if patient antibodies interfere with viral hemagglutination. For example, rubella immunity testing uses this technique.

1. AGGLUTINATION INHIBITION

2. A modification of the agglutination reaction, called agglutination inhibition,
provides a highly sensitive assay for small quantities of an antigen.
For example, one of the early types of home pregnancy test kits included latex
particles coated with human chorionic gonadotropin (HCG) and antibody to HCG.
 The addition of urine from a pregnant woman, which contained HCG, inhibited
agglutination of the latex particles when the anti-HCG antibody was added; thus the
absence of agglutination indicated pregnancy.
Agglutination inhibition assays can also be used to determine whether an individual
is using certain types of illegal drugs, such as cocaine or heroin.

3. The original home pregnancy test kit employed hapten inhibition to determine the presence
or absence of human chorionic gonadotropin (HCG).
The original test kits used the presence or absence of visible clumping to determine whether
HCG was present.
If a woman was not pregnant, her urine would not contain HCG; in this case, the anti-HCG
antibodies and HCG-carrier conjugate in the
No visible clumping kit would react, producing visible clumping.
If a woman was pregnant, the HCG in her urine would bind to the anti-HCG antibodies, thus
inhibiting the subsequent binding of the antibody to the HCG-carrier conjugate.
Because of this inhibition, no visible clumping occurred if a woman was pregnant.

4. Agglutination inhibition assays are widely used in clinical laboratories to determine
whether an individual has been exposed to certain types of viruses that cause
agglutination of red blood cells.
If an individual's serum contains specific antiviral antibodies, then the antibodies
will bind to the virus and interfere with hemagglutination by the virus.
This technique is commonly used in premarital testing to determine the immune
status of women with respect to rubella virus.
The reciprocal of the last serum dilution to show inhibition of rubella
hemagglutination is the titre of the serum.
A titre greater than 10 (1:10 dilution) indicates that a woman is immune to rubella,
whereas a titre of less than 10 is indicative of a lack of immunity and the need for
immunization with the rubella vaccine

6. APPLICATIONS
Agglutination test is widely applicable in the field of Clinical Microbiology
Blood typing of recipient and donor at the time of blood transfusion.
Helps in the detection of antibody presence and to quantify the amount of antibody
present in the patient’s blood.
Active agglutination helps in serodiagnosis of Toxoplasmosis, brucellosis etc.
Passive agglutination helps in the determination of the Rh-factor
Widely used in techniques like latex and hemagglutination.
Helps to detect the type of antigen and to quantify the amount of antigen present in
the patient’s blood.